Sensitivity Training

A few useful tips help avoid frustration and errors when making sensitive voltage measurements. It's not uncommon for modern electrical measurements to involve extremely small quantities. Test methods that produce good results in such cases can't be done haphazardly. Designers accustomed to gathering quick readings with a multimeter must now think about techniques for highly sensitive measurements. They also must understand the limitations of what they can realistically expect for repeatability, resolution, and stability. Generally, signal levels of less than 1 µV require sensitive measurement techniques. Attempting to measure 123 µV to five digits of resolution (123.45 µV) is certainly a sensitive measurement, because the least significant digit represents 10 nV of resolution. A typical voltage source behaves as if it includes a series resistance, RSource, through which all current from the source flows. This is called source resistance or output resistance. Often, R is less than 10k½ and can be ignored. For example, the source resistance of a thermocouple is usually less than 10 ½ and most sensors have output resistances less than 1k½. But there's a need for special care to avoid excessive loading of the source when a voltage is measured through a high source resistance. In general, accurate low-level measurements require instruments that absorb as little energy as possible from the test circuit. For example, a sensitive voltage measurement might need R to be at least 10,000 times R . In addition, every connector, switch, and relay contact in series with the voltage source has the potential to produce significant and unstable error voltage VError in the measurement circuit. Good system design and instrument selection will minimize such errors. Using an instrument optimized for the type of measurement at hand is a good start. High-quality instruments use special components, conductors, insulation, and assembly techniques that minimize errors